Variable reflectivity mirror assemblies, such as electrochromic mirror assemblies are known and are widely implemented in vehicles. The reflective element of the mirror assemblies often include two substrates or glass elements. The back or outer surface of the second substrate (commonly referred to as the “fourth surface” of the reflective element) may include a silvered coating to provide reflectance of an image. In embodiments where the mirror assembly may include a display, a window may be formed, such as by sand blasting, laser etching or the like, through the silvered coating, such that display information may be transmitted through the window for viewing by the driver. The window provides a highly transmissive, generally spectrally neutral window for the display. However, the window defines an area of the reflective element that no longer has the reflective coating, such that reflectivity is lost in the window area. Therefore, the size and the quantity of displays that can be provided at the mirror reflective element is limited.
It is known to provide a metallic reflective layer on an inward surface of the second substrate of the electrochromic reflective element (commonly known in the art as a “third surface” of the reflective element), such as disclosed in U.S. Pat. No. 3,280,701, which is hereby incorporated herein by reference. An electrochromic medium may be positioned between the metallic layer and a transparent electrically conductive layer on the inward surface of the first substrate (i.e., the “second surface” of the reflective element). However, there are concerns with the electrochromic medium of such mirror assemblies contacting the metallic layer, since chemical and/or electro-chemical attack on the metallic layer may result in corrosion of the metallic layer.
As disclosed in U.S. Pat. No. 5,724,187, which is hereby incorporated herein by reference, a metallic conductive layer may be disposed on the third surface, with a protective layer, such as a transparent semi-conductive layer of indium tin oxide, disposed on the metallic layer. The electrochromic medium is then positioned between the protective layer and a conductive layer on the inward surface of the first substrate. It is preferable for such designs to include an adhesion layer, such as a second transparent semi-conductive layer, such as indium tin oxide, or another metallic layer, between the metallic layer and the inward surface of the second substrate, in order to enhance adhesion of the metallic layer to the second substrate.
In electrochromic mirror assemblies which include a display that may transmit through the substrates of the reflective element, the metallic layer or coating must be thin enough to be transmissive to allow viewing of the display through the metallic coating. It is known to provide a thinner metallic coating in a display area to provide increased transmissivity (but with a consequently reduced reflectivity) only in the display area or areas of the reflective element, such as disclosed in U.S. Pat. No. 6,356,376, which is hereby incorporated herein by reference. However, such designs have layers or coatings that are relatively thin (often less than 150 Å or thereabouts in thickness) and so any variation in metallic layer thickness may lead to a significant variation in light transmission through such thin metallic coatings. Thus, such significantly thin metallic coatings or layers may have a substantially low variability tolerance for the thickness and may require a substantially uniform thickness coating, in order to provide the desired results. Such tolerances and uniformity may be difficult to achieve through sputter coating or other coating processes typically used in the manufacture of such reflective elements. Therefore, such significantly thin metallic coatings may be difficult and costly to manufacture.
An example of a known electrochromic reflective element is shown in FIG. 1. The reflective element includes an electrochromic (EC) medium layer and a metallic reflective layer sandwiched between conductive layers at the front and rear glass substrates. A display is positioned at a rear surface of the rear substrate (the fourth surface of the reflective element). The display emits light through the substrates and layers therebetween so as to be viewable by a person viewing the first surface of the reflective element. Such known reflective elements provide little or no spectrally selective transmission characteristics of visible light, as can be seen with reference to FIG. 1A (which shows the transmissivity of the ITO and silver layers at the rear substrate versus the wavelength of the radiant energy), and may be subject to chemical/electrochemical corrosion through contact with the EC medium.
Sometimes it is desired to have an illumination source and/or a camera or imaging device or sensor at an interior rearview mirror assembly for illuminating and/or capturing images of the interior cabin of the vehicle, such as part of a cabin monitoring system, a driver alertness/drowsiness detection system, an intrusion detection system, a seat occupancy detection system and/or the like. The illumination sources and imaging device, if provided at the interior rearview mirror assembly, are typically positioned around the bezel, chin or eyebrow portion of the mirror casing or at a pod or module associated with the mirror assembly or elsewhere in the vehicle. The illumination sources and imaging device cannot typically be positioned within the mirror casing due to the difficulties encountered in projecting light or illumination through the reflective element to the cabin and allowing light from within the cabin to pass through the reflective element to the imaging device. Typically, such transmissivity of light, even of infrared or near infrared light, through the reflective element may not be achieved utilizing reflective coatings that comprise a metallic layer, such as a thin silver or silver alloy or aluminum or aluminum alloy layer or the like. In such applications, the infrared or near infrared light emitted by the illumination source may reflect back into the cavity of the mirror casing, such that a desired amount of light may not reach the cabin and such that the imaging device may be adversely affected by the reflectant light.
Therefore, there is a need in the art for an electrochromic mirror assembly which provides sufficient reflectivity and sufficient transmissivity to allow for transmission of display information or illumination through the reflective element, and which overcomes the above disadvantages and shortcomings of the prior art.